The invention relates generally to the field of non-invasive imaging. More particularly, the invention relates to detectors for use in non-invasive imaging.
In the fields of medical imaging and security screening, non-invasive imaging techniques have gained importance due to benefits that include unobtrusiveness, ease, and speed. A number of non-invasive imaging modalities exist today. Examples include positron emission tomography (PET) imaging, single photon emission computed tomography (SPECT) imaging, magnetic resonance (MR) imaging, ultrasound imaging and X-ray imaging. In medical and research contexts, these imaging systems are used to image organs or tissues beneath the surface of the skin. A particular modality may be selected based upon the organ or tissue to be imaged, upon the spatial and/or temporal resolution desired, or upon whether structural or functional characteristics are of interest.
In PET and SPECT systems an image is generated based upon the impact of radiation photons (generated by a nuclear decay event) against a scintillator. In response to the impact of the radiation photons, the scintillator emits light, which may in turn be detected by optical sensors. For example, the emitted light may be detected using position sensing avalanche photodiodes (PSAPDs) or other photo detector device, such as a photo multiplier tube (PMT). In the example of the PSAPD, timing resolution may be limited by factors such as large capacitance in the PSAPD, low signal to noise ratio, and slow charge collection due to the presence of a resistive layer that facilitates the determination of impact position. For example, large capacitance and series resistance in the PSAPD tend to increase the rise-time associated with a detection event (measured as the slope of the rising edge of the signal) and the reduced signal-to-noise ratio leads to poor timing resolution.
In general, poor timing resolution is not desired. For example, in PET imaging, the timing interval or “window” used to determine that two detection events result from the same decay event must be sufficiently large to accommodate the entire range of timing possibilities for the detector. In some cases it is desirable to reduce the timing interval so that randomly coincident signals may be more easily discarded. However, this reduction may also lead to actual detection events being disregarded due to poor timing resolution of the detector assembly. In particular, the narrower the timing interval, the more likely that signals arising from the same decay event will be discarded simply due to the poor timing resolution of the detector circuitry, i.e., impacts associated with the same decay event may be erroneously determined to not arise from the same decay event. As a result, data that may be useful in forming a diagnostic or other image may be discarded or ignored, leading to less useful and/or lower quality images.